Cavitation detector

ABSTRACT

To distinguish between cavitation and ambient noise, signals are passed to parallel integrator circuits having different time constants which circuits respectively integrate the ambient noise portion alone or in combination with the cavitation portion, the resulting signals being subtracted to obtain an indication of the presence of cavitation.

ite Sites atet l Morin Sept. 4, 1973 [54] CAVITATION DETECTOR 3,147,4679/1964 Laakmann 340/261 [75] Inventor: Jean-Yves Jules Henri Morin,

Toulon, France Primary ExaminerRichard A. Farley [73] Asslgnee' Franca'sPans France Attorney-Eric H. Waters,J0hn G. Schwartz et al.

[22] Filed: July 16, 1971 [211 App]. No.: 163,299

30 Foreign Application Priority Data [57] ABSTRACT July 17, 1970 France7026355 To distmguish between cav1tat1on and amb1ent noise, 52 us. c1.340/5 R, 328/165 Signals are Passed Parallel integrator circuits having51 1m. (:1. H04b 11/00 different time Constants which circuitsrespectively [58] Field of Search 340/2, 5 R, 6 R; tegrate the ambientnoise P ahme combina- 328/165 tion with the cavitation portion, theresulting signals being subtracted to obtain an indication of the pres-[56] References Cited ence of cavltation.

UNITED STATES PATENTS 3,617,998 11/1971 Freedman 340/5 R 5 Claims, 1Drawing Figure Pkf-AMPZ i i L x SUB. x/ro/rapyo/wi/ H.475? 3 C/RCu/T 7PATENTEnsv'ams i h. u ibukb H M @bw CAVITATION DETECTOR FIELD OFINVENTION This invention relates to cavitation detectors and moreparticularly to detectors capable of distinguishing between ambientnoise and cavitation.

BACKGROUND It is known that liquid media may be subjected to thephenomenon of cavitation as a result of the occurrence within the liquidof localized and temporary pressure variations due, for example, to thepresence of a mechanical oscillator or a rotating body. This is thecase,

for example, with the cavitation of the propellers of ships, the bladesof hydro-electric turbines and electroacoustic transducers.

This phenomenon is generally detrimental since it causes considerabledamage to materials and losses in performance of the apparatus in thevicinity of which this phenomenon appears. Therefore, it is important tobe able to detect it and possibly, to appreciate its magnitude.

It is known that cavitation consists of the formation of bubbles of gasin a liquid which implode almost immediately after they form. As aresult there is an increase in the level of noise occurring in theliquid, the term noise designating in this case a chance fluctuation ofpressure.

The methods and apparatus for detecting cavitation which are known areall based on measuring the variation of the mean level of ambient noise.This is characterized by many disadvantages. More particularly, themeasurement of this level is not indicative of the magnitude of thecavitation, since it depends on the mean level of other noises.Moreover, it may result in confusion as to the origin of the increasedmean noise level. Thus these known devices cannot distinguish noise dueto the cavitation of a body which is moving relatively to a liquid wherethe hydrostatic pressure is relatively low from that, in the absence ofcavitation, of a body which is in rapid relative movement in a liquidwhere the hydrostatic pressure is considerable.

An object of the present invention is to obviate the above disadvantagesand render the detection and evaluation of the phenomenon of cavitationindependent of noise or chance fluctuations in pressure occurring in aliquid medium in the absence of cavitation.

For this purpose, an electric voltage delivered by a pressure pickup(also called a transducer) is distributed over two channels where it issubjected to detection and integration, the integration time constant ofone of the channels being determined so as to be responsive only tonoise in the absence of cavitation, whereas that of the other isdetermined so as to be responsive to the existing total noise, that isto say in particular to cavitation. The noise due to cavitation, as isknown, manifests itself in short pressure variations of considerableamplitude, with a steep front. The frequency of repetition and theamplitude of these variations make it possible to characterize theamount of the cavitation. They increase at the same time as the cause ofthe phenomenon (for example, increase in the rotational speed of thepropellers of a ship). The noise, in the absence of cavitation, isgenerally present in different form. The sudden variations in pressureare less frequent and the frequency spectrum is more regular.Furthermore, the amplitudes of the sudden variations in pressure due tocavitation are generally greater by I0 to 20 decibels than those of themean level of permanent noise in the absence of cavitation.

BRIEF DESCRIPTION OF THE DRAWING The invention will be better understoodfrom the following description of one particular non-limitativeapplication in the form of a preferred but non-exclusive embodimentconsidered in conjunction with the accompanying drawing in which thesole FIGURE is a schematic diagram of a circuit of the invention.

DETAILED DESCRIPTION The illustrated circuit is a detector for thecavitation caused by the rotation of the propellers of a ship, forexample a submarine. As shown in FIG. 1 a hydrophone 11 is situated inthe vicinity of the cavitation source. It transforms the pressurefluctuations of the water into fluctuations of electric voltage. It hasa sensitivity, for example, of 96 decibels (ref. 1 volt per bar).

It is followed:

by a pre-amplifier 2 having a high input impedance (greater than 600kiloohms) with an adjustable gain whose maximum value is 50 decibels(ref. 1 volt) by a filter 3 whose band-pass (for 3 decibels ofattenuation) range is between 1 and 20 kilohertz by an amplifier 4 of 40decibels gain which acts on two parallel channels each comprising asquare-law detector 5 by integrators 6a and 6b, each detector beingfollowed by integrator 6a or 6b one of which has a time constant of onehundredth of a second and the other a time constant of a thousandth of asecond by subtraction circuit 7, the output voltages of the twointegrators being subtracted from one another in the circuit 7 and theirdifference being introduced at the inputs of two amplifiers 8a and 8bwhich act on triggers having different voltage thresholds, causing thelighting-up of two pilot lights 9a and 9b one of which indicates slightcavitation and the other considerable cavitation.

In the absence of cavitation, the difference in electric voltage betweenthe two detection-integration channels is constant whatever the level ofnoise at the input (noise of the sea and of the ship in the presentcase) over a Wide variation in this level (up to 20 decibels variation).

If there is cavitation, the short pulses of the corresponding noise aredetected by the channel which has the shortest integration timeconstant. The voltage obtained at the output of the integrator is allthe greater in proportion as the amplitude of these pulses issubstantial and their repetition frequency high.

What is claimed is:

ll. Apparatus for detecting, in a liquid medium, cavitation in thepresence of ambient noise which tends to obscure the same, saidapparatus comprising means to transduce ambient noise and cavitationproduced incidental to rotation of the propeller of a ship into anelectrical signal, first and second parallel integrator means to receivesaid signal and having different time constants such that the integratormeans are adapted respectively for integrating the ambient noise aloneand the ambient noise with the cavitation to provide two outputvoltages, means to compare said voltages to determine the presence ofcavitation, and a filter between the first said means and integratormeans and having a ages.

4. Apparatus as claimed in claim 3 wherein the indicator means includetriggers.

5. Apparatus as claimed in claim 1 wherein said means to compare saidvoltages with a reference voltage.

1. Apparatus for detecting, in a liquid medium, cavitation in thepresence of ambient noise which tends to obscure the same, saidapparatus comprising means to transduce ambient noise and cavitationproduced incidental to rotation of the propeller of a ship into anelectrical signal, first and second parallel integrator means to receivesaid signal and having different time constants such that the integratormeans are adapted respectively for integrating the ambient noise aloneand the ambient noise with the cavitation to provide two outputvoltages, means to compare said voltages to determine the presence ofcavitation, and a filter between the first said means and integratormeans and having a band-pass range of from 1 to 20 kilohertz, the saidtime constants being about one hundreth and one thousandth of a secondrespectively.
 2. Apparatus as claimed in claim 1 wherein said means tocompare said voltages includes a subtraction circuit.
 3. Apparatus asclaimed in claim 1 comprising indicator means coupled to said means tocompare said voltages.
 4. Apparatus as claimed in claim 3 wherein theindicator means include triggers.
 5. Apparatus as claimed in claim 1wherein said means to cOmpare said voltages with a reference voltage.